A head mount display device (HMD), also known as glass display device, video glasses, portable cinema, or the like, is a display equipment directly worn in front of eyes of a user in a manner similar to a helmet or glasses. As shown in FIG. 1, the head mount display device comprises a display unit 1 (such as a liquid crystal display) for generating a display image; preferably there are two display units 1, which respectively provide different display images for the left eye 8a and the right eye 8b of the user, to achieve 3D display; of course, it is also feasible if there is only one display unit 1 that provides a display image for one eye (the left eye 8a or the right eye 8b) or two eyes of the user. The display unit 1 is preferably located at a side portion (such as in a “glass leg”) of the head mount display device, and an display image emitted by the display unit can reach a user's eye (the user's eye present in the text refers to the left eye 8a of the user and/or the right eye 8b of the user) after being reflected by a reflection unit (including a series of reflectors 21 and 22), which is mainly for saving space of the head mount display device; of course, it is also feasible to provide the display unit 1 directly in front of the user's eye without providing reflectors.
Due to volume limitation of the head mount display device, the display image can only be emitted from a place very close to the human eye, and the object distance of the display image (the distance between the display image and the human eye) is very small. To see an object clearly, the following should be satisfied: 1/u+1/v=1/f, wherein u is an object distance; f is the focal distance of a human eye (adjustable within a certain range); and v is an image distance (the object can be seen clearly only when the image distance is equal to the distance from the crystalline lens to the retina). Therefore, when the object distance is very small, the object can be seen clearly only if the focal distance of the human eye is also very small, but this can cause eyestrain and poor viewing experience and harm eyesight. For this reason, the head mount display device should also include a focusing lens unit, which is used for changing the diverging degree of the light rays emitted by the display unit 1 (i.e. the light rays of a display image), thereby “simulating” the display image as “a virtual display image (a virtual image)” emitted from far away (i.e. changing the object distance of the display image). The focusing lens unit 3 is arranged in the propagation path of the light rays of the display image, and each focusing lens unit 3 comprises one or more focusing lenses, and in addition to adjusting the object distance of the display image, the focusing lens unit 3 also has the effects of amplifying the display image, flipping the display image and the like.
Of course, the above display unit 1, reflectors, focusing lens unit 3 and the like have various known forms, and are not described herein in details.
Head mount display devices can be divided into two categories; closed type and open type. As to the closed-type head mount display device, the area in front of the user's eye is closed, and the user can only see the display image, but cannot see an external object. As to the open-type head mount display device, the area in front of the user's eye is at, least partially light-transmissive (a reflector 22, if located here, is preferably a reflection-transmission mirror), so that the user can see both the display image and the external object (such as information on the external object).
As the focal, distance of the human eye at the same moment is fixed, the object distance of the display image of the open-type head mount display device (i.e. the distance from the “virtual display image” to the user's eye) is the same as the object distance of the external object; otherwise, when the focal distance of the user's eye is matched with the object distance of the display image, the display effect is not good, and the display image is clear, but the external object corresponding to the display image is obscure; and when the focal distance of the user's eye is matched with the object distance of the external object, the external object is clear, but the display image is obscure. As the object distance of the external object is unchangeable, it is only possible to adapt the object distance of the display image to the object distance of the external object. The adjustment of the object distance of the display image is achieved by adjusting the position of the focusing lens unit 3 in the light ray propagation direction of the display image (the unidirectional arrow direction between the display unit 1 and the reflector 21 in FIG. 1), which may be overall adjustment of the position of the focusing lens unit 3 or be adjustment of the position of part of the focusing lenses in the focusing lens unit 3. Changing the object distance of the display image by adjusting the position of the focusing lens can be achieved in various known ways, and thus is not described herein in details. Currently, the position of the focusing lens is manually adjusted, and the user adjusts the position of the focusing lens unit 3 by himself through turning a rotary knob or the like.
The inventor finds that at least the following problems exist in the prior art: when an open-type head mount display device is used, an external object seen by the user changes at all times, accordingly the object distance of the external object also changes at all times, and this requires the user to adjust the object distance of the display image from time to time, so the operation is troublesome; and manual adjustment is not likely to be performed timely, so the phenomenon of image obscurity still occurs, and the display effect is not good.